1. Field of the Invention
The present invention relates to methods for correcting the outputs of inertial navigation devices for thermally-induced bias errors. More particularly, this invention pertains to a method that utilizes a thermal model calibration that is readily amenable to in-field updating on a continuing basis.
2. Description of the Prior Art
Aircraft inertial navigation relies upon the integration of data throughout a sequence that begins when the aircraft is prepared for takeoff and ends when the aircraft has landed and motion ceased. The inertial navigation system ("INS") of an aircraft includes various components, including accelerometers and gyroscopes, that convert the effects of inertial forces into acceleration, velocity and position measurements. The accelerometers determine acceleration forces along three orthogonal sensitive axes and this data is converted, through integrations, into the aircraft's velocity and position. In a strapdown system in which the accelerometer is fixed in a relation to the geometry of an aircraft, the gyroscopes that measure the aircraft's attitude also measure that of the accelerometer axes. Data measured by the gyros is employed to resolve accelerometer outputs along the appropriate spatially stabilized axes.
Error sources that affect the accuracy of the gyro and accelerometer outputs require compensation to ensure accuracy of the navigation system measurements and functions. Systems and instruments come in various forms and rely upon disparate technologies to produce outputs. Gyroscopes may include gimballed mechanical or electromechanical arrangements, ring laser and fiber optic arrangements, among others, while accelerometers can be of the pendulous mass type and/or employ ring laser, fiber optic, piezoelectric or silicon technologies. Regardless, each inertial navigation system arrangement is faced, to a greater or lesser extent, with inaccuracies owing to the bias error peculiarities of its functional components. It is well known, for example, that the bias error of a ring laser gyroscope of the multioscillator type is strongly dependent upon temperature. The overall modelled bias error of the device includes a temperature-insensitive term that may be treated as constant and both linear and sinusoidal functions of temperature.
Because inertial grade instruments are required to measure a very large dynamic range of motions, they typically rely on state-of-the-art technologies. These sensors must be able to measure extremely small quantities. For example, a navigation grade accelerometer must measure a few millionths of the standard gravity acceleration, and a gyro must measure a few hundred millionths of the Earth's rotation rate. Often, it is impossible to precisely identify the sources of minute errors of these magnitudes. Instead, the instruments are typically corrected for global parameters such as temperature. This type of correction requires a model which is presumed to be repeatable.
When an INS is calibrated, the behaviors of its instruments' bias errors as functions of temperature are routinely modelled. This is done in recognition of the fact that the instruments will face and be required to operate over a range of temperatures when in field use. Such a range is an inevitable consequence of off-on operation as well as variations in location, altitude and cooling air temperature.
The instrument model is then employed by the inertial navigation system to compensate instrument outputs. Bias error characteristics will generally undergo perceptible changes over the long term. For example, it is possible to model or calibrate the bias-error-versus temperature characteristic of a ring laser gyroscope of the multioscillator type at the factory over a maximum temperature range. This model may then be employed in operational software to compensate for predicted errors. Nevertheless, the gyro thermal model can gradually shift during hundreds of hours of operation to such an extent that the model is degraded. Eventually, the INS may be returned to the factory or other remote facility for calibration or remodeling of its temperature-sensitive component devices.
The mean time before repair ("MTBR") of an INS is thus affected by drift of the modelled thermal bias errors of navigation devices. This has led to past efforts to devise systems and methods for updating the thermal bias error models of navigation devices while in field use. Approaches have included minibiasing at the end of initial alignment, a process described by J. Diesel, in "Calibration of a Ring Laser Gyro Inertial Navigation System For Minimum Velocity Error," Fourteen Biennial Guidance Test Symposium, Central Inertial Guidance Test Facility, Guidance Test Division, 6585th Test Group, Holloman AFB vol. II (Oct. 3-5, 1989) at pages 1--1 through 1-20. Other attempts have involved post-flight updating, and updating using GPS. All of such methods have focused upon adjustment of the temperature-independent term and each method is therefore inherently of only limited utility. As time goes on, the effects of sensor aging and resulting of thermal model degradation upon bias errors become more pronounced.